344 THE RESPIRATORY SYSTEM 



cupula pleurae, so that the same line can be used to indicate 

 both the apex of the lung and the pleural cupula on the surface 

 of the body. This line commences at the junction of the 

 medial and middle thirds of the clavicle, arches upwards and 

 medially, and then descends to reach the sterno-clavicular joint. 

 At its highest point, it lies not more than i inch above the 

 clavicle. 



The line along which the costal becomes continuous with 

 the mediastinal pleura is known as the costo-mediastinal line 

 of reflection. Its position differs slightly on the two sides of 

 the body. On the right side, it begins at the sterno-clavicular 

 articulation and passes downwards and medially to the middle 

 of the manubrium. From this point it descends vertically till 

 it reaches the level of the sixth chondro-sternal joint, which 

 corresponds to the lower limit of the mediastinal pleura on 

 the anterior surface of the body. On the left side, the upper 

 part of the line is similar, but, opposite the fourth chondro- 

 sternal articulation, it passes laterally to the margin of the 

 sternum, along which it descends to the sixth costal cartilage 

 (PL II.). 



The lines of the two sides, therefore, overlap one another 

 from the middle of the manubrium to the level of the fourth 

 costal cartilage, but their upper and lower extremities are 

 separated by small intervals. The V-shaped interval behind 

 the upper part of the manubrium overlies the origins of the 

 innominate and left common carotid arteries and the trachea, 

 while in the lower interval the pericardium comes into direct 

 apposition with the posterior surface of the sternum (p. 290). 

 Light percussion over these areas gives a dull note, in conse- 

 quence of these relations. 



At the lower limit of the pleural sac, the costal pleura is 

 reflected on to the upper surface of the diaphragm. The 

 line along which this reflection takes place is not horizontal, 

 but inclines downwards as it is traced laterally (PI. II.). 

 The costo-diaphragmatic line of reflection begins at the 

 lower end of the costo-mediastinal line and passes down- 



